In electronic devices and systems, proper device or system operation is often dependent, at least in part, upon correct timing. Typically, a timing module which includes such components as a clock source and counters or dividers provides timing or clock signals to other electronic components. In the event that the clock signals are not at an expected frequency or within a particular tolerance thereof, then the components which rely on the clock signals, and thus the electronic device or system, might not operate as intended.
It is therefore generally desirable to provide for effective monitoring of the frequency of clock signals used in electronic equipment. Although clock signals represent perhaps the most common type of periodic electrical signal for which accurate frequencies are critical, there may be other types of periodic electrical signal for which effective frequency monitoring is important.
U.S. Pat. No. 6,707,329, issued on Mar. 16, 2004 and entitled “CLOCK RECOVERY AND DETECTION OF RAPID PHASE TRANSIENTS”, describes a typical example of comparing a subject clock signal to a reference signal to determine whether the subject clock signal is within a specified frequency tolerance of the reference signal. According to the above patent, a high and a low counter are driven by the subject clock signal when its phase lags or leads the reference signal, respectively. Each counter has a corresponding threshold, the crossing of which by the counter's count causes an alarm to be triggered. The techniques described in the above patent, however, are intended primarily for clock recovery and phase transient detection, and thus might not be feasible for implementation for the purposes of periodic electrical signal frequency monitoring in general. In addition, these techniques require that one input signal, which is not monitored itself, be used as a reference signal for another input signal.